This invention relates to bandpass filters suitable for use generally at microwave frequencies.
Bandpass filters are widely used in microwave systems, for example in signal generating systems to remove spurious signals outside a desired frequency band and in signal detecting systems to prevent over-loading by signals outside the desired band and to remove other undesired signals such as image-frequency signals produced in mixers.
Known microwave bandpass filters can be categorised by the type of transmission line in which they are formed. One common kind is coupled-line filters formed in strip transmission line, comprising a cascade of half-wavelength portions of line, one half of each portion being edge-coupled to the preceding portion and the other half to the succeeding portion. Although such filters can be made to cover band-widths up to about an octave (see IEEE Transactions on Microwave Theory and Techniques, MTT-29, pp. 215-222 (March 1981)), the widths of the (lowest-frequency) passband and the stopband immediately above it are inevitably limited by the fact that the centre frequency of the next-higher passband is three times the centre frequency of the lowest passband. Moreover, they cannot provide very high selectivity, and tend to be rather long.
A pair of known kinds of bandpass filter closely related to one another are respectively of combline and capacitively-loaded interdigital structure. Methods of designing such filters for arbitrary desired bandwidths has been proposed by R. J. Wenzel in "Synthesis of Combline and Capacitively Loaded Interdigital Bandpass Filters of Arbitrary Bandwidth", IEEE Transactions on Microwave Theory and Techniques, MTT-19, No. 8 (August 1971), pp. 678-686. While such filters are significantly smaller than previous filters of the same line structure, they have the disadvantages that they are expensive, are not readily reproducible (nominally identical filters require a plurality of tuning screws for adjustment to meet the same performance specification), and are unsuitable for high selectivity (with combline, particularly at the lower edge of the lowest-frequency passband).
A further kind of bandpass filter is formed in coaxial line. The disadvantages of such filters include inability to provide high selectivity at the lower end of the passband, and a significant length if a moderately strict performance specification is to be met.